UKPassats

hi all,have noticed over the last couple of weeks a louder noise from pulleys quite hard to describe,a dry whirring?!!also hard to pinpoint if it top or bottom,equally loud above and below.have read a few posts about alternator pulley,tensioners but not sure where to start,nothing seems to be affected by it.
any one had this problem,and if so how did you go about identifying the offending item?
all help and suggestions appreciated,adam